The invention relates to a method for determining the status of a plurality of switches. The invention also relates to an apparatus for determining the status of a plurality of switches.
In aircraft engine control systems, various switches are used to feed back different aspects of engine and/or airframe condition, the engine control system commonly being controlled by an electronic control unit (ECU). The closing of a switch against a respective switch contact causes a current to pass through the closed switch path to indicate, for example, an overload or limit, or an action by the aircraft pilot. During operation of the engine control system, there is often a need to be able to determine whether the switches are in an open or closed position.
Conventionally, switch status is determined by means of a circuit, forming part of the electronic control unit, which periodically supplies a xe2x80x9cwettingxe2x80x9d current to all of the external switches in the control system by means of an arrangement of primary switches. The wetting current is supplied to each switch via an associated resistor by closing the primary switches. The electrical circuit includes an arrangement of secondary switches, one secondary switch being associated with each of the external switches. By closing the secondary switches, the current flowing through the closed secondary switch path provides an indication of the external switch status.
In order to prevent oxidisation and debris build-up on the external switch contacts, it is necessary to ensure a current of at least 4 mA is supplied to the closed external switch contacts. Oxidisation and debris build-up on the switch contacts can occur for smaller currents than this and results in a degradation in performance and reliability of the external switches.
However, with larger currents flowing through the closed external switches, a significant power loss occurs across the internal resistors of the ECU. This gives rise to undesirable heating effects which can adversely effect the thermal management of the electronic control unit and can degrade the performance and reliability of overheat protection circuitry contained therein. A problem therefore exists with the conventional method of determining switch status in that a compromise must be made between the most desirable wetting current for reduced heat loss and the most desirable wetting current for reliable switch operation.
It is an object of the present invention to provide a method and apparatus for determining switch status which alleviates the problems of the prior art.
According to one aspect of the present invention, there is provided a method for determining the status of a plurality of switches in a circuit, each switch having a first switching means for controlling the supply of a wetting current to the respective switch, and second switching means, closing of the second switching means enabling the status of the respective switch to be determined, the method including the steps of closing the first switching means associated with each of the switches so as to supply a wetting current to each of the switches and, when each one of the first switching means is closed, closing the second switching means associated with the same switch in order to determine switch status.
Preferably, the duration for which each pair of the first and second switching means is closed is chosen so as to minimise power dissipation.
Conveniently, the method of the present invention may be used to determine whether each of the switches in the circuit is open or closed. The duration is chosen to be sufficient to accurately determine the switch status. Preferably, the duration for which each pair of the first and second switching means is closed is between 50 xcexcs and 1 ms and, typically, may be 500 xcexcs.
In a preferred embodiment, the wetting current is supplied to each switch in sequence, the status of the switches being determined sequentially.
The method provides the advantage that, as the wetting current is only supplied to the switches in the circuit as a pulse of short duration rather than continuously, heat losses due to resistances within the circuit are reduced. Thus, higher wetting currents can be used than with conventional methods. It has been found that this improves the performance and reliability of the method.
A further improvement is obtained by supplying the wetting current to the switches sequentially rather than at the same time.
Additionally, the method may include the further steps of reading the status of each switch and storing it in the form of a single bit in a register when the associated second switching means is closed and, prior to closing the first and second switching means associated with the next switch in the sequence, shifting the contents of the register by one register location.
Preferably, the method may include the further step of outputting the contents of the register when the register contains the status of all of the switches. Alternatively, the ECU may be arranged to access the data stored in the register at any desired time.
The method provides the further advantage that, as the status of each of the switches is determined one after the other, a simplified read-out register arrangement is required. Conventionally, if the status of all the external switches is determined at the same time, a read-out register is required for each switch.
The switches may form part of an engine and/or airframe control function. However, the method may be used for determining the status of switches in any circuit.
The first and second switching means preferably take the form of first and second multiplexer switch arrangements, the first multiplexer switch arrangement comprising a plurality of first switches and the second multiplexer switch arrangement comprising a plurality of second switches.
According to a second aspect of the present invention, there is provided an apparatus for determining the status of a plurality of switches in a circuit, comprising;
a first switching means associated with each switch for controlling the supply of a wetting current to the respective switch;
a second switching means associated with each switch, whereby closing of the second switching means enables the status of the respective switch to be determined; and
means for addressing the first and second switching means so as to switch the first and second switching means between open and closed positions such that the first and second switching means associated with a common switch are closed at substantially the same time.
Preferably, the first and second switching means associated with a common switch are closed at substantially the same time, for a period of relatively short duration.
Preferably, the pairs of first and second switching means associated with each switch are operated sequentially.
The apparatus may also include a register into which the status of each of the switches is output. The contents of the register can conveniently be read-out when one sequence has been completed and the status of all of the switches has been determined.
Preferably, the apparatus comprises a first multiplexer switch arrangement and a second multiplexer switch arrangement, the first multiplexer switch arrangement comprising a plurality of first switches, each one of the first switches being operable to control the supply of a wetting current to an associated one of the switches, the second multiplexer switch arrangement comprising a plurality of second switches, each of the second switches being operable to enable the status of an associated switch to be determined.